Polarizing plate and flexible display device including the same

ABSTRACT

A flexible display device includes a display panel configured to display an image, and a polarizing plate on the display panel. The flexible display device includes a folding area and a non-folding area. A first thickness of the polarizing plate in the folding area is greater than a second thickness of the polarizing plate in the non-folding area.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. § 119(a) of KoreanPatent Application No. 2017-0126432, filed in Korea on Sep. 28, 2017,which is incorporated herein by reference in its entirety.

BACKGROUND 1. Technical Field

The present disclosure relates to a display device, and moreparticularly, to a flexible display device.

2. Discussion of the Related Art

Recently, as an information society progresses, the field of a displaydevice capable of processing and displaying a large amount ofinformation has been rapidly developed. For example, the development ofvarious flat panel display devices has been in the spotlight.

Examples of a flat panel display device include a liquid crystal display(LCD) device, a plasma display panel (PDP) device, a field emissiondisplay (FED) device, an electroluminescence display (ELD) device, anorganic light emitting diode (OLED), and the like. These flat paneldisplay devices may include excellent performance in reduction ofthickness, weight, and power consumption, in contrast with related artcathode ray tube (CRT) displays, which are being rapidly replaced withsuch flat panel display devices.

Meanwhile, the flat panel display device may use a glass substrate towithstand high-temperature heat generated during the manufacturingprocess, so there may be a limitation in providing a light weight,thinness, and flexibility.

For this reason, a flexible display device, which is manufactured usinga flexible material such as a plastic instead of a glass substratewithout flexibility to maintain the display performance if bent likepaper, has been in the spotlight as a next-generation flat panel displaydevice.

The flexible display device may utilize a thin film transistor substrateinstead of glass, and may be classified into an unbreakable flexibledisplay device having high durability, a bendable flexible displaydevice, a rollable flexible display device, a foldable flexible displaydevice, etc. Such a flexible display device may have advantages in spaceutilization and interior decoration and design, and may be used invarious application fields.

For example, in order to realize a large-sized display device having anultra-thin thickness, a light weight, and a compact structure, thebendable or foldable display device, which can be carried in a foldedstate and display an image in an unfolded state, has been activelystudied.

The bendable or foldable display device (hereinafter referred to as a“foldable display device”) may be applied in various fields such as a TVand a monitor as well as a mobile device such as a mobile phone, anultra-mobile PC, an electronic book, an electronic newspaper, or thelike.

However, because the flexible display device may need to be folded orunfolded, the flexible display device may be provided in the form of athin film. Impact resistance of such a flexible display device may beextremely weak.

For example, a polarizing plate may be drawn to have a stretchingratio—e.g., a ratio of the amount the polarizing plate is drawn(stretched) relative to its original, unstretched amount. A polarizingplate that is drawn to a maximum stretching ratio (e.g., stretched to amaximum amount before breaking) may frequently generate cracks in afolding area of the flexible display device.

Because the polarizing plate having cracks may degrade the displayquality of the display panel, the polarizing plate having cracks may bediscarded by removing the polarizing plate from the flexible displaydevice. However, when it is difficult to remove the polarizing platefrom the flexible display device, the flexible display device may bediscarded.

Thus, the process efficiency and/or the production yield may bedegraded.

SUMMARY

Accordingly, the present disclosure is directed to an emitting diode andan electroluminescent display device that substantially obviate one ormore of the problems due to limitations and disadvantages of the relatedart.

An object of the present disclosure is to provide a polarizing platecapable of preventing a crack.

Another object of the present disclosure is to provide a flexibledisplay device having high image quality.

Additional features and advantages of the disclosure will be set forthin the description which follows, and in part will be apparent from thedescription, or may be learned by practice of the disclosure. Theobjectives and other advantages of the disclosure will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the disclosure, as embodied and broadly described herein, there isprovided a flexible display device, comprising a display panelconfigured to display an image; and a polarizing plate on the displaypanel, wherein the flexible display device includes a folding area and anon-folding area; and a first thickness of the polarizing plate in thefolding area is greater than a second thickness of the polarizing platein the non-folding area.

In another aspect, a flexible display device comprises a display panelconfigured to display an image; and a polarizing plate on the displaypanel and having a first stretching ratio in a folding area and a secondstretching ratio in a non-folding area, wherein the second stretchingratio is greater than the first stretching ratio.

In another aspect, a polarizing plate for a flexible display devicecomprises a polarizing layer having a first stretching ratio in a firstarea and a second stretching ratio in a second area; and first andsecond TAC films on respective sides of the polarizing layer, whereinthe second stretching ratio is greater than the first stretching ratio.

It is to be understood that both the foregoing general description andthe following detailed description are explanatory, and are intended toprovide further explanation of the embodiments as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the disclosure, are incorporated in and constitute apart of this application, illustrate implementations of the disclosureand together with the description serve to explain the principles ofembodiments of the disclosure. In the drawings:

FIGS. 1A and 1B are perspective views showing a folded state and anunfolded state of a flexible display device according to a firstembodiment of the present disclosure.

FIG. 2A is an enlarged sectional view showing the folded state of theflexible display device according to the first embodiment of the presentdisclosure.

FIG. 2B is an enlarged sectional view showing the unfolded state of theflexible display device according to the first embodiment of the presentdisclosure.

FIG. 2C is a sectional view showing a display panel of a flexibledisplay device according to the first embodiment of the presentdisclosure.

FIGS. 3A and 3B are sectional views showing a polarizing plate accordingto the first embodiment of the present disclosure.

FIG. 4 is a sectional view showing a polarizing plate according to asecond embodiment of the present disclosure.

FIG. 5 is a simulation result obtained by measuring a difference invisibility between a folding area and a non-folding area of a flexibledisplay device according to the second embodiment of the presentdisclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of the presentdisclosure, examples of which are illustrated in the accompanyingdrawings.

FIGS. 1A and 1B are perspective views showing a folded state and anunfolded state of a flexible display device according to a firstembodiment of the present disclosure.

A foldable display device is shown as an example of a flexible displaydevice 100 in the first embodiment of the present disclosure. However,the present disclosure is not limited thereto, and may be applied tovarious display devices (DD) such as a curved display device, a bendabledisplay device, a rollable display device, and a stretchable displaydevice.

Although not shown, the flexible display device 100 according to thefirst embodiment of the present disclosure may be applied to small tomedium-sized electronic devices such as a cellular phone, a personalcomputer, a notebook computer, a personal digital terminal, a carnavigation unit, a game machine, a portable electronic appliance, awristwatch-type electronic device, a camera, and the like as well as alarge-sized electronic device such as a television or an externaladvertisement board.

As shown in FIGS. 1A and 1B, the flexible display device 100 includes aplurality of areas that are defined on a display surface. The flexibledisplay device 100 may be divided into a display area (or “emissionarea”) EA and a non-display area (or “non-emission area”) NEA dependingon whether an image is displayed. The display area EA is configured todisplay an image and the non-display area NEA is adjacent to the displayarea EA and does not display an image.

The non-display area NEA is located along edges of the display area EA.The flexible display device 100 may have a folding area FA, which isfolded along the folding axis FX according to an action, as well as afirst non-folding area NFA1 and a second non-folding area NFA2, whichare not folded.

FIGS. 2A and 2B are enlarged sectional views showing a folded state andan unfolded state of the flexible display device according to the firstembodiment of the present disclosure, and FIG. 2C is a sectional viewshowing a display panel of the flexible display device.

As shown in FIG. 2A, the flexible display device 100 may be folded alongthe folding axis FX such that a display surface of the first non-foldedarea NFA1 faces a display surface of the second non-folded area NFA2.

As shown in FIGS. 2A and 2B, the flexible display device 100 includes adisplay panel 110 for displaying an image, a touch panel 120 includingtouch sensors (not shown), a back plate 130 for supporting the displaypanel 110, and a cover window 140 for protecting the display panel 110.

For the sake of convenience, it will be defined on the basis of thedrawing that the back plate 130 is positioned on a rear surface of thedisplay panel 110 and the cover window 140 is positioned forward of thedisplay panel 110 when the display surface of the display panel 110 isdirected forward. In addition, the touch panel 120 is positioned betweenthe display panel 110 and the cover window 140.

The display panel 110 may be formed of one of a liquid crystal display(LCD) device, a plasma display panel (PDP) device, a field emissiondisplay (FED) device, an electroluminescence display (ELD) device, andan organic light emitting diode (OLED). In an example, the OLED, whichis representative of a flexible display device capable of maintainingdisplay performance even if it is bent like paper, is used as thedisplay panel 110.

The OLED is a self-luminous element and does not require a backlight asused in a liquid crystal display device, which itself is a non-emissiveelement. Thus, the OLED may have a light weight and a slim structure.

In addition, in comparison to a liquid crystal display device, the OLEDmay be superior in terms of a viewing angle, a contrast ratio, powerconsumption, a direct current (DC) low-voltage driving, and a responsespeed. Further, because internal components of the OLED are formed in asolid, the OLED may be strong against an external impact and may have awide operating temperature range

Because the manufacturing process is simple, the OLED may remarkablyreduce the manufacturing cost when compared to a liquid crystal displaydevice.

In the display panel 110 formed of the OLED, a substrate 101 formed witha driving thin film transistor DTr and a light emitting diode E isencapsulated with a protective film 102.

Hereinafter, the display panel 110 (formed of the OLED (herein referredto as an OLED panel) will be described in more detail with reference toFIG. 2C.

A plurality of pixel regions P are defined on the display area EA formedon the substrate 101. A semiconductor layer 104 is formed in each pixelregion P, and the semiconductor layer 104 is formed of silicon. Anactive area 104 a constituting a channel is formed at the center of thesemiconductor layer 104 and source and drain areas 104 b and 104 c dopedwith impurities at high concentration are formed on both sides of theactive area 104 a.

A gate insulating layer 105 is formed on the semiconductor layer 104. Agate electrode 107 and a gate line (not shown), which extends in onedirection, are formed on the gate insulating layer 105 in correspondencewith the active area 104 a of the semiconductor layer 104.

In addition, a first interlayer insulating layer 106 a is formed on theentire top surface of the gate electrode 107 and the gate line (notshown). In addition, the first interlayer insulating layer 106 a and thegate insulating layer 105 formed under the first interlayer insulatinglayer 106 a have first and second semiconductor layer contact holes 109for exposing the source and drain areas 104 b and 104 c located on bothsides of the active area 104 a, respectively.

Source and drain electrodes 108 a and 108 b, which are spaced apart fromeach other and are in contact with the source and drain areas 104 b and104 c exposed through the first and second semiconductor layer contactholes 109, respectively, are formed on the first interlayer insulatinglayer 106 a including the first and second semiconductor layer contactholes 109.

In addition, a second interlayer insulating layer 106 b having a draincontact hole 112 for exposing the drain electrode 108 b is formed on thesource and drain electrodes 108 a and 108 b and the first interlayerinsulating layer 106 a exposed between the source and drain electrodes108 a and 108 b.

In this case, the source and drain electrodes 108 a and 108 b, thesemiconductor layer 104 including the source and drain areas 104 b and104 c in contact with the source and drain electrodes 108 a and 108 b,the gate insulating layer 105 formed on the semiconductor layer 104, andthe gate electrode 107 may constitute the driving thin film transistorDTr.

Data lines (not shown) which cross gate lines (not shown) are providedto define the pixel region P. A switching thin film transistor (notshown) may have the same structure as the driving thin film transistorDTr and is connected to the driving thin film transistor DTr.

In FIG. 2C, as an example, the switching thin film transistor (notshown) and the driving thin film transistor DTr are shown as a co-planartype transistor, in which the semiconductor layer 104 is formed of apolysilicon semiconductor layer. Alternatively, the switching thin filmtransistor and the driving thin film transistor DTr may be provided as abottom gate type transistor in which the semiconductor layer 104 isformed of pure and impure amorphous silicon.

In addition, a first electrode 111 is formed on a predetermined area ofthe second interlayer insulating layer 106 b where the image is actuallydisplayed. For example, the first electrode 111 serves as an anode of alight emitting diode E and is formed of a material having a relativelyhigh work function value.

The first electrode 111 is connected to the drain electrode 108 b of thedriving thin film transistor DTr, and the first electrode 111 is formedin each pixel region P. A bank 119 is interposed between the firstelectrodes 111 formed in each pixel region P.

That is, the bank 119 may serve as a boundary between the pixel regionsP so that the first electrodes 111 formed in the pixel regions P may beisolated from each other by the bank 119.

In addition, an organic light emitting layer 113 is formed on the firstelectrode 111.

The organic light emitting layer 113 may be configured as a single layerformed of a light emitting material or may be configured as a multiplelayer including a hole injection layer, a hole transport layer, a lightemitting layer, an electron transport layer, and an electron injectionlayer, in order to improve light emitting efficiency.

In addition, a second electrode 115 serving as a cathode is formed onthe entire surface of the organic light emitting layer 113.

The second electrode 115 has a dual layer structure and includes asemitransparent metal film, which is formed by thinly depositing a metalmaterial having a low work function. In this case, the second electrode115 may have the dual layer structure in which a transparent conductivematerial is thickly deposited on the semitransparent metal film.

Accordingly, light emitted from the organic light emitting layer 113 isdirected to the second electrode 115, which is called a top emissiontype method.

Alternatively, the second electrode 115 may be formed of an opaque metalfilm and the light emitted from the organic light emitting layer 113 maybe directed to the first electrode 111, which is called a bottomemission type method.

In the OLED panel 110, when a predetermined voltage is applied to thefirst electrode 111 and the second electrode 115 in response to aselected color signal, holes injected from the first electrode 111 andelectrons provided from the second electrode 115 are transported to theorganic light emitting layer 113 to form excitons. When the excitonstransition from the excited state to the ground state, light isgenerated and emitted in the form of visible light.

In this case, the emitted light passes through the transparent secondelectrode 115 or the first electrode 111 and exits to the outside, sothat the OLED panel 110 displays an image.

The protective film 102 in the form of a thin film is formed on thedriving thin film transistor DTr and the light emitting diode E so thatthe OLED panel 110 is encapsulated by the protective film 102. In orderto prevent external oxygen and moisture from penetrating into the OLEDpanel 110, at least two inorganic protective films are laminated andused. In this case, an organic protective film may be interposed betweenthe two inorganic protective films to reinforce the impact resistance ofthe inorganic protective film.

Meanwhile, the substrate 101 may be formed of a thin polyimide to have aflexible property. However, the substrate 101 formed of the thinpolyimide may not be suitable for the process of forming a componentsuch as the thin film transistor DTr, so the process of forming thecomponent such as the thin film transistor DTr may be performed in astate in which a substrate formed of a polyimide is attached to acarrier substrate such as a glass substrate. The OLED panel 110 may beobtained by separating the carrier substrate from the polyimidesubstrate.

A touch panel 120 is disposed on the OLED panel 110. Although not shown,the touch panel 120 includes a first touch film (not shown) having afirst touch electrode (not shown) and a second touch film (not shown)having a second touch electrode (not shown), which are spaced apart fromeach other while facing each other.

The first touch electrode (not shown) is deposited on an entire surfaceof the first touch film (not shown), and the first touch electrode (notshown) is formed of a transparent conductive material such as indium tinoxide (ITO) or indium zinc oxide (IZO).

In addition, the second touch electrode (not shown) is formed on thesecond touch film (not shown) facing the first touch film (not shown).The second touch electrode is provided in a bar shape to be spaced apartfrom each other by a predetermined interval and is formed of a metalmaterial such as aluminum (Al) or an aluminum alloy (AlNd), magnesium(Mg), gold (Au), silver (Ag), or the like.

The first and second touch electrodes (not shown) may constitute a touchsensor (not shown).

Accordingly, when a predetermined input device such as a finger or a pentouches an upper portion of the first touch film (not shown), the firsttouch electrode (not shown) formed on the first touch film (not shown)and the second touch electrode (not shown) formed on the second touchfilm (not shown) are electrically connected, so that a voltage value ofthe touch point may be changed according to a resistance value of thetouch point. Thus, a control device may find position coordinates basedon the variation of the potential difference.

The touch panel 120 and the OLED panel 110 are integrally modularizedthrough the cover window 140 and the back plate 130. The touch panel 120and the cover window 140 for protecting the OLED panel 110 are disposedat a front surface of the OLED panel 110 that displays an image, and theback plate 130 for supporting the OLED panel 110 is disposed at a rearsurface of the OLED panel 110.

The cover window 140 protects the OLED panel 110 and the touch panel 120from an external impact and transmits light emitted from the OLED panel110 so that the image displayed on the OLED panel 110 is visible to theoutside.

The cover window 140 may be formed of a polymeric material having impactresistance and light transmittance properties, such aspolymethylmethacrylate (PMMA), polycarbonate (PC), a cycloolefin polymer(COP), polyethylene terephthalate (PET), a polyimide (PI), or apolyaramid (PA).

In addition, because the substrate 101 of the OLED panel 110 is verythin, the back plate 130 is attached to the rear surface of the OLEDpanel 110 to support the OLED panel 110.

Meanwhile, because the display panel 110 is formed of an OLED in theflexible display device 100 according to the first embodiment of thepresent disclosure, a polarizing plate 200 is disposed at an outersurface of the OLED panel 110, through which light is transmitted, inorder to prevent the contrast from being lowered due to external light.

That is, when the OLED panel 110 is in the driving mode for displayingan image, the polarizing plate 200 for blocking the external lightincident from the outside is positioned in a transmission direction oflight emitted through the organic light emitting layer 113, therebyimproving the contrast.

The polarizing plate 200 is a circular polarizing plate for blockingexternal light. Thus, the flexible display device 100 according to thefirst embodiment of the present disclosure may reduce and/or minimizethe reflection of the external light by using the polarizing plate 200,thereby preventing the contrast from being lowered.

In the flexible display device 100 according to the first embodiment ofthe present disclosure, a linear polarizing plate 210 (see FIG. 3A) ofthe polarizing plate 200 is drawn 2 times to 5 times (e.g., 2 to 5 timesits original amount) in a folding area FA, and is drawn 6 times (e.g., 6times its original amount), which is the maximum stretching ratio, infirst and second non-folding areas NFA1 and NFA2.

The stretching ratio will be described below in more detail.

With reference to FIG. 2B, the OLED panel 110 and the touch panel 120are combined with each other through a first optical adhesive layer 150a, the touch panel 120 and the polarizing plate 200 are combined witheach other through a second optical adhesive layer 150 b, and thepolarizing plate 200 and the cover window 140 are combined with eachother through a third optical adhesive layer 150 c. In addition, theOLED panel 110 and the back plate 130 are combined with each otherthrough a fourth optical adhesive layer 150 d so that the flexibledisplay device 100 is integrally modularized.

The first to fourth optical adhesive layers 150 a, 150 b, 150 c and 150d are formed of an optically clear adhesive (OCA) and may have athickness in the range of 100 μm to 300 μm.

When the first to fourth optical adhesive layers 150 a, 150 b, 150 c and150 d have a thickness of 100 μm or less, the adhesive force is so weakthat the cover window 140, the back plate 130, the polarizing plate 200,the touch panel 120, and the OLED panel 110 may not be integrallymodularized. When the first to fourth optical adhesive layers 150 a, 150b, 150 c and 150 d have a thickness of 300 pin or more, the flexibledisplay device 100 may not be easily folded.

As described above, in the flexible display device 100 according to thefirst embodiment of the present disclosure, the polarizing plate 200provided to prevent the contrast from being lowered due to externallight is divided into the folding area FA and the non-folding areas NFA1and NFA2. The polarizing plate 200 is drawn 2 times to 5 times in thefolding area FA, and is drawn 6 times—which is the maximum stretchingratio—in the non-folding areas NFA1 and NFA2, thereby preventing cracksfrom being generated in the folding area FA when the flexible displaydevice 100 is folded or unfolded.

As a result, the display quality of the flexible display device 100 maybe prevented from degrading, and the process efficiency can be improvedbecause it may not be necessary to remove the polarizing plate havingcracks.

FIGS. 3A and 3B are sectional views showing the polarizing plateaccording to the first embodiment of the present disclosure.

As shown in FIGS. 3A and 3B, the polarizing plate 200 includes aretarder 220 and a linear polarizing plate 210 positioned on theretarder 220. With regard to the arrangement of the linear polarizingplate 210 and the retarder 220, the linear polarizing plate 210 may bearranged closely to an incident direction of external light, and theretarder 220 is arranged inward of the linear polarizing plate 210.

The linear polarizing plate 210 includes a polarizing layer 211 forchanging the polarizing property of light and first and second cellulosetriacetate (TAC) films 213 a and 213 b formed on both sides of thepolarizing layer 211 to protect and support the polarizing layer 211.

The linear polarizing plate 210 absorbs linearly polarized lightparallel to an absorption axis of the polarizing layer 211 and transmitslinearly polarized light perpendicular to the absorption axis, that is,the linearly polarized light parallel to a transmission axis.

The polarizing layer 211 may be formed of poly-vinyl alcohol (PVA),which is drawn and coated with iodine ions or dichroic dyes, in whichthe polarizing layer 211 is drawn 2 times to 5 times in the folding areaFA, and is drawn 6 times, which is the maximum stretching ratio, in thenon-folding areas NFA1 and NFA2.

That is, the polarizing plate 200 has the folding area FA, which isfolded to form a curvature when the flexible display device 100 (seeFIG. 2B) is folded, and the non-folding areas NFA1 and NFA2, which areformed at both sides of the folding area FA and maintain a flat statewhen the flexible display device 100 (see FIG. 2B) is folded.

In this case, the polarizing layer 211 is drawn 2 times to 5 times inthe folding area FA and is drawn 6 times, which is the maximumstretching ratio, in the non-folding areas NFA1 and NFA2.

In this manner, the stretching ratio in the folding area FA is lowerthan the stretching ratio in the non-folding areas NFA1 and NFA2, sothat a thickness of the polarizing layer 211 in the folding area FA isgreater than a thickness of the polarizing layer 211 in the non-foldingareas NFA1 and NFA2.

In this case, because the polarizing layer 211 in the folding area FA isthicker than the polarizing layer 211 in the non-folding areas NFA1 andNFA2, the first and second TAC films 213 a and 213 b located on bothsides of the polarizing layer 211 may have a step between the foldingarea FA and the non-folding areas NFA1 and NFA2 corresponding to thethickness of the polarizing layer 211.

In addition, the retarder 220 positioned on the linear polarizing plate210 is a quarter wave plate (QWP) having a phase retardation value of¼λ.

Further, the retarder 220 also has a step between the folding area FAand the non-folding areas NFA1 and NFA2 corresponding to the step of thesecond TAC film 213 b.

Therefore, in the overall thickness of the polarizing plate 200, whenthe folding area FA has a first thickness ‘a’, the non-folding areasNFA1 and NFA2 have a second thickness b smaller than the first thickness‘a’.

Accordingly, the flexible display device 100 (see FIG. 2B) according tothe first embodiment of the present disclosure may prevent cracks frombeing generated in the polarizing plate 200 in the folding area FA whenthe flexible display device 100 is folded or unfolded.

That is, the polarizing plate 200 may be drawn at the maximum stretchingratio to improve the degree of polarization, and in an example, themaximum stretching ratio of the polarizing plate 200 is 6 times.

However, when the polarizing plate 200 is folded after stretching thepolarizing plate 200 at the maximum stretching ratio, an additionalstretching may not occur inside the polarizing plate 200, so that thetensile stress generated in the neutral plane (NP) exceeds a criticalpoint, thereby generating cracks such as tearing in the polarizing plate200.

In contrast, because the polarizing plate 200 of the flexible displaydevice 100 (see FIG. 2B) according to the first embodiment of thepresent disclosure is configured to draw the polarizing layer 211 2times to 5 times in the folding area FA, an additional stretching mayoccur inside the polarizing layer 211.

Thus, even when the polarizing plate 200 is folded, the tensile stressgenerated in the neutral plane of the polarizing plate 200 is less thanthe critical point, so that cracks can be prevented from being generatedin the polarizing plate 200.

As a result, it may be possible to prevent the display quality of theflexible display device (100 of FIG. 2B) from being lowered, and theprocess efficiency may be improved because it is not necessary to removethe polarizing plate having cracks.

The first and second thicknesses ‘a’ and ‘b’ of the polarizing plate 200in the folding area FA and the non-folding areas NFA1 and NFA2 maysatisfy the following Equation 1.

a={(b−z)/6}x+z   Equation 1

In Equation 1, ‘z’ is an initial thickness before stretching, and ‘x’ isa stretching ratio in the folding area FA.

When the polarizing layer 211 in the non-folding areas NFA1 and NFA2 hasthe second thickness b of 10 μm under the conditions that the initialthickness of the polarizing layer 211 before stretching is 30 μm and thepolarizing layer 211 is stretched at the maximum stretching ratio of 6times, the polarizing layer 211 in the folding area FA has the firstthickness ‘a’ of 13.4 μm when stretched 5 times, 16.7 μm when stretched4 times, 20 μm when stretched 3 times, and 23.3 μm when stretched 2times.

Therefore, in an example, the ratio a/b of the first thickness ‘a’ tothe second thickness ‘b’ satisfies the following Equation 2.

1(Min)<a/b≤2.3(Max)   Equation 2

With reference to Table 1 below, the drawn state may not be stablymaintained due to the elastic restoring property when the polarizinglayer 211 is drawn 2 times or less in the process of stretching thepolarizing layer 211, so that the polarizing layer 211 is substantiallydrawn 2 times or less.

TABLE 1 First thickness Second thickness Stretching ratio (folding area)(non-folding area) a/b 6 times   10 μm 10 μm 1 5 times 13.4 μm 10 μm 1.34 times 16.7 μm 10 μm 1.7 3 times   20 μm 10 μm 2 2.5 times   23.3 μm 10μm 2.3

Therefore, in the flexible display device 100 (see FIG. 2B) according tothe first embodiment of the present disclosure, the first thickness ‘a’of the polarizing plate 200 in the folding area FA may not exceed 23 μm,and the polarizing plate 200 is drawn 2 times to 5 times to have thestretching ratio lower than the maximum stretching ratio to satisfyEquation 2.

Particularly, the ratio a/b of the first and second thicknesses ‘a’ and‘b’ in the folding area FA and non-folding areas NFA1 and NFA2 of thepolarizing plate 200 of the flexible display device 100 (see FIG. 2B)according to the first embodiment of the present disclosure may satisfythe following Equation 3.

1(Min)<a/b≤1.3(Max)   Equation 3

That is, since the polarizing performance may be improved as thestretching ratios of the folding area FA and the non-folding areas NFA1and NFA2 become more similar, the folding area FA may be designed suchthat the difference between the stretching ratio of the folding area FAand the maximum stretching ratio of the non-folding areas NFA1 and NFA2is as small as possible.

Therefore, the folding area FA may be drawn 5 times, which is similar tothe stretching ratio of the non-folding areas NFA1 and NFA2 drawn at themaximum drawn ratio, and in an example, the first thickness ‘a’ isdesigned to be 13.4 μm closest to the second thickness b.

When the stretching ratio of the folding area FA is set to 5 times, thefolding area FA has an additional stretching margin of 17% as comparedwith the non-folding areas NFA1 and NFA2. Therefore, even if thepolarizing plate 200 is folded, the tensile stress generated in theneutral plane of the polarizing plate 200 is less than the criticalpoint, so cracks may be prevented from being generated in the polarizingplate 200.

As a result, it may be possible to prevent the display quality of theflexible display device 100 (see FIG. 2B) from being lowered, and theprocess efficiency may be improved because it is not necessary to removethe polarizing plate having cracks.

In addition, the folding area FA having the first thickness ‘a’ that isthicker than the second thickness ‘b’ of the non-folding areas NFA1 andNFA2 has a step difference with respect to the non-folding areas NFA1and NFA2, and the step difference may satisfy the following Equation 4with reference to Table 2 below.

TABLE 2 First thickness Second thickness Stretching ratio (folding area)(non-folding area) a/b d (=(a − b)/2) 5 times 13.4 μm 10 μm 1.3 1.7 μm 4times 16.7 μm 10 μm 1.7 3.4 μm 3 times   20 μm 10 μm 2 5.0 μm 2.5times   23.3 μm 10 μm 2.3 6.7 μm

With reference to Table 2, the step difference ‘d’ between the foldingarea FA and the non-folding areas NFA1 and NFA2 may be set in the rangeof 6.7 μm to 1.7 μm. Actually, the step difference may be variously setdepending on the stretching ratios of the folding area FA and thenon-folding areas NFA1 and NFA2 within the thickness of the second andthird optical adhesive layers 150 b and 150 c (see FIG. 2B) disposed onboth sides of the polarizing plate 200.

Because the optical adhesive layers 150 a, 150 b, 150 c and 150 d of theflexible display device 100 according to an embodiment of the presentdisclosure have an ultra-thin thickness of 25 μm or less, the stepdifference between the folding area FA and the non-folding areas NFA1and NFA2 may be defined as shown in the following Equation 4.

1.7 μm<d≤25 μm   Equation 4

As described above, in the flexible display device 100 (see FIG. 2B)according to the first embodiment of the present disclosure, thepolarizing layer of the polarizing plate 200 is drawn 2 times to 5 timesin the folding are FA and is drawn 6 times—which is the maximumstretching ratio—in the non-folding areas NFA1 and NFA2, therebypreventing cracks from being generated in the polarizing plate 200 inthe folding area FA when the flexible display device 100 (see FIG. 2B)is folded or unfolded.

As a result, it may be possible to prevent the display quality of theflexible display device 100 (see FIG. 2B) from being lowered, and theprocess efficiency maybe be improved because it is not necessary toremove the polarizing plate having cracks.

FIG. 4 is a sectional view showing the polarizing plate according to asecond embodiment of the present disclosure.

In order to avoid redundant descriptions, the same parts as those of thefirst embodiment described above will be denoted by the same referencenumerals.

As shown in FIG. 4, a polarizing plate 200 according to the secondembodiment of the present disclosure includes a retarder 220 and alinear polarizing plate 210 positioned on the retarder 220, and thelinear polarizing plate 210 includes a polarizing layer 211 for changingthe polarizing property of light and first and second TAC films 213 aand 213 b formed on both sides of the polarizing layer 211 to protectand support the polarizing layer 211.

The linear polarizing plate 210 absorbs linearly polarized lightparallel to an absorption axis of the polarizing layer 211 and transmitslinearly polarized light perpendicular to the absorption axis, that is,the linearly polarized light parallel to a transmission axis.

The polarizing layer 211 may be formed of poly-vinyl alcohol (PVA),which is drawn and coated with iodine ions or dichroic dyes, in whichthe polarizing layer 211 is drawn 2 times to 5 times in the folding areaFA, and is drawn 6 times—which is the maximum stretching ratio—in thenon-folding areas NFA1 and NFA2.

In this manner, the stretching ratio in the folding area FA is lowerthan the stretching ratio in the non-folding areas NFA1 and NFA2, sothat a thickness of the polarizing layer 211 in the folding area FA isgreater than a thickness of the polarizing layer 211 in the non-foldingareas NFA1 and NFA2.

In this case, because the polarizing layer 211 in the folding area FA isthicker than the polarizing layer 211 in the non-folding areas NFA1 andNFA2, the first and second TAC films 211 and 221 located on both sidesof the polarizing layer 211 may have a step between the folding area FAand the non-folding areas NFA1 and NFA2 corresponding to the thicknessof the polarizing layer 211.

In addition, the retarder 220 positioned under the linear polarizingplate 210 is a quarter wave plate (QWP) having a phase retardation valueof ¼λ.

Further, the retarder 220 also has a step between the folding area FAand the non-folding areas NFA1 and NFA2 corresponding to the step of thesecond TAC film 213 b.

Therefore, in the overall thickness of the polarizing plate 200, whenthe folding area FA has a first thickness ‘a’, the non-folding areasNFA1 and NFA2 have a second thickness ‘b’ smaller than the firstthickness ‘a’.

The polarizing plate 200 according to the second embodiment of thepresent disclosure further includes a surface treatment layer 230positioned on the linear polarizing plate 210, that is, on the first TACfilm 213 a.

The surface treatment layer 230 is formed of an anti-glare layerincluding a silica bead (not shown) and has a haze characteristic.

The haze characteristic refers to a phenomenon in which, when lightpasses through a transparent material, an opaque blurred image appearsdue to light diffusion caused by the intrinsic properties of thetransparent material other than reflection or absorption, and lighttransmittance becomes higher as the haze characteristic value islowered.

The surface treatment layer 230 according to the second embodiment ofthe present disclosure is configured to have an external hazecharacteristic in the range of 10% to 11% and an internal hazecharacteristic in the range of 20% to 33%. The external hazecharacteristic refers to a haze characteristic realized by the surfacetreatment layer 230 itself and the internal haze characteristic refersto a haze characteristic realized by the silica bead (not shown)contained inside the surface treatment layer 230.

In this case, the total haze characteristic due to the external hazecharacteristic and the internal haze characteristic may be set to 20% ormore.

Because the surface treatment layer 230 having the haze characteristicis provided outside the polarizing plate 200, the difference invisibility between the folding area FA and the non-folding areas NFA1and NFA2 may be reduced and/or minimized in the unfolded state of theflexible display device 100 (see FIG. 2B).

That is, in the polarizing plate 200 according to the second embodimentof the present disclosure, the stretching ratio in the folding area FAis different from the stretching ratio in the non-folding areas NFA1 andNFA2 in order to prevent cracks from being generated in the folding areaFA when the flexible display device 100 (see FIG. 2B) is folded, so adifference in visibility may occur when the flexible display device 100(see FIG. 2B) is unfolded due to the difference in the degree ofpolarization between the folding area FA and the non-folding areas NFA1and NFA2.

However, as described above, because the surface treatment layer 230having the haze characteristic is provided outside the polarizing plate200, it may be possible to reduce and/or minimize the difference invisibility between the folding area FA and the non-folding areas NFA1and NFA2 in the unfolded state of the flexible display device 100 (seeFIG. 2B) as shown in FIG. 5.

In this case, the surface treatment layer 230 may be provided in a coverwindow 140 (see FIG. 2B) positioned at the outermost side of theflexible display device 100 (see FIG. 2B). The surface treatment layer230 may have an external haze characteristic of 10% to 23% and aninternal haze of 2% to 33% when the surface treatment layer 230 isprovided in the cover window 140.

In this case, the total haze characteristic due to the external hazecharacteristic and the internal haze characteristic may be set to 20% ormore.

As described above, in the flexible display device 100 (see FIG. 2B)according to the second embodiment of the present disclosure, thepolarizing layer 211 of the polarizing plate 200 is drawn 2 times to 5times in the folding area FA and is drawn 6 times—which is the maximumstretching ratio—in the non-folding areas NFA1 and NFA2, therebypreventing cracks from being generated in the polarizing plate 200 inthe folding area FA when the flexible display device 100 (see FIG. 2B)is folded.

As a result, it may be possible to prevent the display quality of theflexible display device 100 (see FIG. 2B) from being lowered, and theprocess efficiency may be improved because it is not necessary to removethe polarizing plate having cracks

Particularly, because the surface treatment layer 230 having the hazecharacteristic is provided outside the polarizing plate 200, thedifference in visibility due to the difference in the degree ofpolarization between the folding area FA and the non-folding areas NFA1and NFA2 may be reduced and/or minimized in the unfolded state of theflexible display device 100 (see FIG. 2B).

Meanwhile, although it has been illustrated and described that the touchpanel 120 (see FIG. 2B) is positioned on the OLED panel 110 (see FIG.2B) for displaying an image in the flexible display device (see FIG.2B), the touch panel 120 (see FIG. 2B) may be omitted.

Although the OLED panel 110 (FIG. 2B) has been described as an exampleof the display panel in the present disclosure, a liquid crystal panelmay be used as the display panel. When the liquid crystal panel is usedas the display panel, the polarizing plate 200 according to theembodiment of the present disclosure may be positioned on both sides ofthe liquid crystal panel.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the embodiments of thepresent disclosure without departing from the spirit or scope of thedisclosure. Thus, it is intended that the present disclosure cover themodifications and variations of this disclosure provided they comewithin the scope of the appended claims and their equivalents.

What is claimed is:
 1. A flexible display device, comprising: a displaypanel configured to display an image; and a polarizing plate on thedisplay panel, wherein the flexible display device includes a foldingarea and a non-folding area; and a first thickness of the polarizingplate in the folding area is greater than a second thickness of thepolarizing plate in the non-folding area.
 2. The flexible display deviceof claim 1, further comprising: a back plate on a surface of the displaypanel; and a cover window on another surface of the display panel,wherein the polarizing plate is between the display panel and the coverwindow.
 3. The flexible display device of claim 2, further comprising: atouch panel between the polarizing plate and the display panel, whereinthe display panel and the touch panel are combined with each otherthrough a first optical adhesive layer; the touch panel and thepolarizing plate are combined with each other through a second opticaladhesive layer; and the polarizing plate and the cover window arecombined with each other through a third optical adhesive layer.
 4. Theflexible display device of claim 1, wherein the display panel is anorganic light emitting display panel.
 5. The flexible display device ofclaim 1, wherein: the first and second thicknesses of the polarizingplate satisfy the equation a={(b−z)/6}x+z, where ‘a’ is the firstthickness, ‘b’ is the second thickness, ‘z’ is an initial thicknessbefore stretching, and ‘x’ is a stretching ratio in the folding region.6. The flexible display device of claim 1, wherein: the ratio of thefirst thickness to the second thickness is 1 to 2.3.
 7. The flexibledisplay device of claim 1, wherein: the polarizing plate is a circularpolarizing plate for blocking external light.
 8. The flexible displaydevice of claim 1, further comprising: a surface treatment layer on thepolarizing plate, wherein the surface treatment layer is formed of ananti-glare layer and has a haze characteristic.
 9. The flexible displaydevice of claim 1, the polarizing plate further comprising: a polarizinglayer; and first and second TAC films formed on respective sides of thepolarizing layer, wherein the polarizing layer in the folding area isthicker than the polarizing layer in the non-folding area; and the firstand second TAC films have a step between the folding area and thenon-folding area corresponding to the difference in thickness of thepolarizing layer in the folding area and the non-folding area.
 10. Aflexible display device, comprising: a display panel configured todisplay an image; and a polarizing plate on the display panel and havinga first stretching ratio in a folding area and a second stretching ratioin a non-folding area, wherein the second stretching ratio is greaterthan the first stretching ratio.
 11. The flexible display device ofclaim 10, further comprising: a back plate disposed under the displaypanel.
 12. The flexible display device of claim 10, wherein: the firststretching ratio is 2-5 times, and the second stretching ratio is 6times.
 13. The flexible display device of claim 10, wherein: a firstthickness ‘a’ of the polarizing plate in the folding area is greaterthan a second thickness ‘b’ of the polarizing plate in the non-foldingarea.
 14. The flexible display device of claim 13, wherein: a stepdifference of the polarizing plate between the first thickness ‘a’ andthe second thickness ‘b’ is in the range of 1.7 μm to 25 μm.
 15. Theflexible display device of claim 10, further comprising: a surfacetreatment layer on the polarizing plate, wherein the surface treatmentlayer has a haze characteristic including an external hazecharacteristic and an internal haze characteristic, wherein the externalhaze characteristic is lower than the internal haze characteristic. 16.The flexible display device of claim 15, wherein: the external hazecharacteristic is 10% to 11%, and the internal haze characteristic is20% to 33%.
 17. The flexible display device of claim 2, furthercomprising: a surface treatment layer on the cover window, wherein thesurface treatment layer has a haze characteristic including an externalhaze characteristic of 10% to 23% and an internal haze characteristic of2% to 33%.
 18. A polarizing plate for a flexible display device,comprising: a polarizing layer having a first stretching ratio in afirst area and a second stretching ratio in a second area; and first andsecond TAC films on respective sides of the polarizing layer, whereinthe second stretching ratio is greater than the first stretching ratio.19. The polarizing plate of claim 18, wherein: the polarizing layer hasa first thickness in the first area and a second thickness in the secondarea, and the second thickness is smaller than the first thickness. 20.The polarizing plate of claim 18, wherein a ratio of the first thicknessto the second thickness is 1 to 2.3.
 21. The polarizing plate of claim18, further comprising: a retarder under the second TAC film.
 22. Thepolarizing plate of claim 18, further comprising: a surface treatmentlayer having an external haze characteristic and an internal hazecharacteristic, wherein the external haze characteristic is lower thanthe internal haze characteristic.
 23. The polarizing plate of claim 22,wherein: the external haze characteristic is 10% to 11%, and theinternal haze characteristic is 20% to 33%.